I've recently purchased the iZotope Iris sampler plugin. While experimenting with the visual spectrum editor, I find that sometimes the drawing tools appear to do nothing. It's as if I'm only alotted so many edits to the image, and I have run out. However, I cannot find any indication of what this limit is or why it happens.

Is this a known limitation in Iris? Have I missed something in the interface or instructional material?

Some experimenting suggests that I can draw again if I undo previous edits, or clear my edits out entirely.